This Week in Science

Science  17 Nov 2006:
Vol. 314, Issue 5802, pp. 1045
  1. Damaged, Detained, But Undeterred


    A robot may operate autonomously in controlled environments, but in new or dangerous terrain, it may become stuck or damaged. Robots are often preprogrammed to deal with specific situations, but this precaution does not help with unexpected events. Bongard et al. (p. 1118; see the Perspective by Adami) have constructed a robotic system that can sense and recover from damage to its structure without prior programming. The robot creates an internal model of its structure that is continually updated to account for change.

  2. Linear Materials, Nonlinear Heat Flow

    Electrical rectifiers allow current to flow in one direction, but it would seem that devices that could rectify thermal energy and direct heat flow would violate Fourier's Law. However, Peierls noted more than 50 years ago that in one dimension, heat flow can be anomalous, and recent theoretical work has suggested that rectification could be possible, albeit experimentally challenging. Chang et al. (p. 1121; see the news story by Service) report a rectification effect on the order of a few percent in which either carbon or boron nanotubes are given an axial asymmetry by the creation of a gradient on high-mass organoplatinum molecules at one end. The authors attribute the rectification effects to heat being carried by solitons.

  3. P, B, and H2


    Many transition metal compounds can reversibly add and eliminate H2, but compounds of lighter elements tend not to undergo this reaction sequence cleanly, as a result of both unfavorable bonding thermodynamics and poor orbital alignment for efficient kinetics. Welch et al. (p. 1124; see the Perspective by Kubas) find that an air-stable phosphonium borate compound with P-H and B-H bonds cleanly liberates H2 above 100°C, and efficiently adds it back upon exposure in solution to the gas at room temperature. The reaction is unusual in that dimesitylphosphine adds to the para carbon of a phenyl ring in tris(pentafluorophenyl)borane, rather than binding more conventionally to the boron center. The reaction may have implications for the development of relatively light-weight substances for storage and release of hydrogen.

  4. Cold Snap

    The shattering of rock by ice freeze has long been thought to be caused by volumetric expansion when water distributed within the rock freezes. However, Murton et al. (p. 1127; see the Perspective by Hallet) demonstrate experimentally an alternative mechanism, called ice segregation, which operates when there is a temperature gradient. As the freezing front moves through the rock, it squeezes water from its pores into pockets where ice lenses form, which causes the rock to crack. Cold-room experiments quantified this process by monitoring heave, temperature, moisture, and pore-pressure for two distinct thermal regimes. The results are verified with numerical modeling and are consistent with field observations. In warming climates, such fracturing may increasingly destabilize permafrost in polar regions.

  5. Neon Puzzle Solved

    Noble gas isotope ratios in lunar soils differ from those of the solar wind, and the explanation given has been that the lunar soils recorded a second component of energetic solar noble-gas particles that may have been stronger in the past but that is not now identifiable. Grimberg et al. (p. 1133) measured how neon in the solar wind decomposes when caught in glass detectors on the Genesis spacecraft, and they observed a change in isotope ratio with depth of implantation caused by fractionation. This process can explain the variation seen on the Moon's surface without recourse to other mechanisms.

  6. Neanderthal Metagenomics

    Our understanding of Neanderthal biology and culture remains limited. These extinct hominids are thought to have been genetically distinct from the human lineage. Noonan et al. (p. 1113; see the news stories by Pennisi and Balter) have now obtained sufficient amounts of Neanderthal genomic sequence, based on sequencing of nuclear DNA from a 38,000-year-old specimen, to create a metagenomic library. They find that humans and Neanderthals shared a common ancestor up to ∼706,000 years ago and that the populations split ∼370,000 years ago.

  7. Making RNA, One Molecule at a Time

    In the initial steps of transcription, RNA polymerase (RNAP) binds to promoter DNA and engages in abortive cycles of synthesis and release of short RNA products until it escapes the promoter and enters processive RNA synthesis. How RNA translocates relative to DNA in the initial transcribing complex has been controversial, with three models proposed (see the Perspective by Roberts). Now two single-molecule studies, one using fluorescence-energy transfer, by Kapanidis et al. (p. 1144), and the other using DNA nanomanipulation, by Revyakin et al. (p. 1139), show that initial transcription involves “scrunching,” in which RNAP remains fixed on the promoter and downstream DNA into itself. Accumulated stress from DNA scrunching stress could thus provide the driving force for both abortive initiation and for promoter escape and productive initiation.

  8. Incorporating Sugars After Protein Folding

    N-linked protein glycosylation is the most frequent posttranslational modification of proteins in eukaryotic cells, and a functionally homologous process also occurs in bacteria. The key component of this bacterial system is PglB, an oligosaccharylotransferase that catalyzes the transfer of the oligosaccharide to selected asparagine residues within a protein. Kowarik et al. (p. 1148) show that, unlike the eukaryotic system, the bacterial oligosaccharyl transferase can act independently of the protein translocation machinery and can glycosylate fully folded proteins in vitro.

  9. Improving Polio Vaccine Efficacy

    Critics of current plans to eradicate poliovirus have questioned whether eradication is feasible. Of the four remaining countries where polio is endemic, India represents perhaps the greatest challenge to global eradication because transmission continues despite massive immunization efforts. Grassly et al. (p. 1150) use disease surveillance data collected since 1997 to show that high population density and poor sanitation are causing persistence by facilitating the transmission of poliovirus and by severely compromising the efficacy of the live-attenuated vaccine. Switching to the monovalent form of the vaccine could potentially provide increased efficacy and allow eventual eradication.

  10. Sorting, Signaling, and Sara


    Morphogenic gradients of signaling molecules are key to tissue patterning during development. Endocytic compartments have been shown to play a role in the generation and maintenance of such gradients. Bökel et al. (p. 1135; see the Perspective by Knoblich) now provide evidence that the apical signaling endosome, characterized by the presence of the protein Sara (Smad anchor for receptor activation), uses the mitotic spindle to distribute developmentally important signaling molecules across division. This process ensures that the two daughter cells retain information contained in the morphogen gradient.

  11. The Psychology Value of Money

    Money can be exchanged for material goods that are essential for our physiological and psychological well-being, but are there direct effects of money on our psychological state and behavior? Vohs et al. (p. 1154; see the Perspective by Burgoyne and Lea) primed human subjects to think about having money and found that these subjects acted in a more self-sufficient fashion than those who were not primed. Possessing money made it less likely that subjects would ask for help in solving a problem, or offer help to another person, or make donations. In addition, subjects with money would distance themselves—literally and figuratively—from others.

  12. Directing the Muscosal Immune Response

    The mucosal lining of the intestine is stuffed with antibody-secreting B cells, which produce vast quantities of immunoglobulin A (IgA); a specialized form of antibody equipped specifically for secretion across the gut wall, where it protects against enteric pathogens. The cues that make a mucosal B cell produce IgA, rather than any of the other forms of antibody, are unclear. Mora et al. (p. 1157) now show that another immune cell, the dendritic cell, imparts this information within lymphoid tissue associated with the gut. Once activated by the gut dendritic cells, B cells become “imprinted” to enter the circulation and then home back to the mucosal lining, to begin IgA production. Induction depended on the vitamin A metabolite retinoic acid, which may explain why vitamin A deficiency exacerbates childhood diarrheal disease in the developing world.

  13. Forest Fires and Global Warming

    Climate warming could cause an increase in boreal forest wildfires, but whether such fires reinforce warming is unclear. Warming could be amplified by increased production of soot, which would decrease surface albedo if it deposits on ice or snow, and by increased CO2 release. Randerson et al. (p. 1130) report measurements and analyses of a boreal fire in Alaska during 1999 and of landscape changes after older fires, and then integrated all of the climate-relevant effects. During the first year after the recent fire, the net impact has been to increase warming, mostly because of the immediate increase of atmospheric CO2 and the effects on ozone and of black carbon and other aerosols. However, they predict that the integrated net forcing during the next 80 years will decrease, mostly because of the sustained increases in surface albedo accompanying forest regrowth and continuing uptake of CO2.

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